1. Field of the Invention
This invention relates to sensing devices and more particularly to a vehicle sensor mechanism for use in a vehicular traffic data accumulating system.
2. Description of the Prior Art
Traffic data accumulating systems are used for gathering various types of information regarding traffic movements such as: vehicular speed, direction, time, for counting purposes, and the like. These systems have traditionally employed a vehicle sensing mechanism which includes an elongated hollow pneumatic road tube which is stretched across one or more traffic lanes in a roadway. The tube is closed on one end and has its other end connected to a diaphragm switch. When the axle of a vehicle passes over the tube, an air pressure wave is generated within the tube, this wave displaces the diaphragm to mechanically operate a set of electrical contacts. A data accumulating device such as a recorder, is connected to the electrical contacts and records each time these contacts close.
This diaphragm switch, or any other similar mechanically operated device, is hereinafter referred to as a digital switch due to the output signal being digital in nature. The digital switches have proven to be the most inaccurate portion of the system due to their inability to accurately respond to the variety of complex air pressure waves created within the pheumatic tube.
The air pressure waves created within the tube are complex in that they are multi-pulse oscillating waves of various amplitudes, pulse widths, and frequencies. Several factors effect the characteristics of these wave forms such as: speed of the traffic, weight of the vehicle, tire width, location along the length of the tube where the vehicle contacts the tube, and wave attenuation.
High speed traffic produces a wave having a high amplitude, short pulse width, and high frequency. Lower speed traffic produces a wave having lower amplitude, longer pulse width, and lower frequency. Heavy vehicles create a wave having higher amplitude than the wave created by a lighter vehicle. Wide tires displace more air within the tube than does a narrow tire.
The attenuation of an air pressure wave increases approximately as the square of the inside diameter of the tube decreases. That is, a tube having an inside diameter of 3/16 inch will attenuate a wave more than a tube of 3/8 inch inside diameter. Other conditions which effect attenuation of the pressure wave are tube length, wall thickness, and tube condition.
A relatively light vehicle which contacts the road tube in the proximity of the digital switch produces a signal which easily operates the switch. When the same vehicle contacts the tube on its other end, the wave may be attenuated along the length of the tube to a point where it is of insufficient amplitude to activate the switch.
Another problem effecting the accuracy of such devices, is waves which may be defined as spurious waves. Under ideal conditions, each axle of the vehicle which contacts the road tube will, for all practical purposes, produce a single legitimate complex air pressure wave. However, as is quite often the case, an axle will produce more than one complex air pressure wave, and all waves over one are considered as spurious waves. One example of spurious waves is sometimes referred to as a reflected wave, that is, one which bounces off of the closed end of the road tube. Another type of spurious pressure wave, which is particularly troublesome at lower vehicular speeds, is one which occurs when all of the wheels of an axle do not contact the road tube at the same time, thus producing two separate complex pressure waves.
It may be apparent from the foregoing brief description of the complexity of air pressure waves and the occurrence of spurious air pressure waves, that vehicular traffic sensor mechanisms must be able to respond to a tremendous variety of legitimate pressure waves and also must be able to distinguish between legitimate and spurious waves in order to achieve any degree of accuracy.
The digital switch type of prior art sensor mechanisms all tend to be inaccurate due to the plurality of individual pulses within a single air pressure wave which cause the switch contacts to chatter. This type of device is also incapable of distinguishing between a legitimate air pressure wave and a spurious wave.
A particular prior art device which alleviated the problems of the complexity of legitimate air pressure waves and of spurious air pressure waves is fully disclosed in U.S. Pat. No. 3,699,398, issued on Oct. 17, 1972 to the same inventor. Briefly, this device, hereinafter referred to as the Analog Self-Adjusting Switch type of prior art device, employs an electromagnetic transducer which senses the air pressure wave produced within the pneumatic road tube and electromagnetically induces a voltage which is coupled to an electronic circuit. This type of device is called an Analog Self-Adjusting Switch due to the characteristics of the induced voltage being determined by the characteristics of the air pressure waves sensed. The electronic circuit blanks out unwanted portions of the voltage and responds to the remaining portion thereof by producing a single output signal for each pressure wave. This output signal is shaped and timed by the electronic circuit so as to be independent of the characteristics of the air pressure wave. This blanking out capability is just as effective in handling the complexity of spurious pressure waves as it is in handling the complexity of legitimate pressure waves. Therefore, the electronic circuit is provided with a signal interrupting means which, after the circuit has produced an output signal, interrupts the ability of the circuit to produce an additional output signal for a predetermined length of time. This interrupting feature prevents voltages induced by spurious pressure waves from producing a second output signal for the same axle.
This Analog Self-Adjusting Switch type of prior art unit was found to be extremely accurarte and it solved the problems of the complexity of the legitimate air pressure waves and the spurious pressure waves when those waves are produced by normal highway traffic where devices of this type are usually employed.
Highway traffic is considered to be normal when the average speed of the vehicles is above 20 miles per hour, with the speeds below that minimum figure being hereinafter referred to as low speed traffic.
Traffic data accumulating systems of these types which are employed to monitor normal traffic movements are usually not employed to monitor low speed traffic movements as monitoring of low speed traffic is difficult and at best very inaccurate. However, a need exists for monitoring of such traffic as would occur, for example, at the drive-in window of some business establishments, such as banks.
The percentage of error is increased in all prior art traffic data accumulating systems of the above types which attempt to monitor low speed traffic.
The Analog Self-Adjusting Switch type of prior art has a problem due to the air pressure waves being of low amplitude, long pulse width and low frequency. Signals of this type are usually too weak to activate this type of sensor and errors result. One method of solving, or at least reducing the percentage of error resulting from these weak signals, is to employ a road tube having a larger inside diameter than is normally used. This results in greater amplitude and less attenuation of signals, and signals having sufficient strength to activate the sensor usually result. This type of large bore road tube cannot be used to sense normal traffic movement as the pressure waves produced thereby will often be strong enough to damage the equipment due to overloading. Therefore, this method of solving the weak signal problem is not practical from a cost or convenience standpoint as the tube must be changed for each type of traffic movement to be monitored. Another method of solving the problem was attempted; namely, increasing the sensitivity of the device to render it capable of reacting to the weak signals. This second method also proved inadequate as overloading also occurred when monitoring of normal traffic was attempted.
The digital type of prior art switch may also experience difficulties with weak pressure waves, however, this problem is solved in this type of unit by adjusting the sensitivity.
The problem of spurious air pressure waves, such as those which occur when the wheels of an axle contact the tube at different times, are compounded in low speed traffic monitoring to a point where even the Analog Self-Adjusting Switch type of prior art device is no longer effective in handling this problem. The length of time between the legitimate air pressure wave and any spurious pressure waves is so great in low speed traffic that the duration of output signal interrupt provided by the signal interrupting means is too short to prevent the spurious waves from producing additional output signals. If this duration is increased in the analog switch type prior art device, then the duration will be too long for normal traffic monitoring purposes.
In view of the foregoing, a need exists for an improved sensor mechanism for use in data accumulating systems which may be employed to monitor either low, medium, or high speed traffic movements.